Different results for eye tests

My son has been on atropine drops (which we put in his eyes every night before bedtime). He has 6 monthly check up and had one last week and the result of 2 hours of dilation and test was that his eyes had stabilized and still were at 3 and 3.5 spherical reading. I went to the optometrist to get him a new set of sports glasses and he underwent testing of his eyes and the reading came back much worse at 4 and 4.5. The last week's appointment was done in the morning (9.30am) while yesterday's was at 4.30pm. I fail to understand how two such tests have significant differing results within a span of a week. Now, my wife believes we should not get the high powered lens as per yesterday's test but my belief is the previous week's test was done by constantly dilating his eyes which is not what happens day-to-day basis, so what he requires is the high powered lens.

Firstly, I would like to understand why the results are so different - do I need a third opinion? What is the correct way forward?